I'm looking at some of the good older threads about the book - it's an interesting question as to why they might have changed things so that Michael went to see Kay when he returned to the States. One possibility is that Kay and Mama had something of a relationship in the book, because Kay occasionally called Mama when Michael was first gone to Sicily. That would have required interrupting the film for chatty phone calls.

More important, though, is that in the book the scene where Michael tells Kay how things are for him now is a sex scene, and that Copolla wouldn't want Michael to be seen in a tender light at that point. Better to have him in a suit and an oversized hat, lumbering down the street with his driver creeping along behind, so that the audience can see that he's a different man now, even if Kay can't see it yet.